1. Field of the Invention
The present invention relates to an oxygen sensor for detecting the concentration of oxygen in exhaust gas from internal combustion engines or other burning devices.
2. Background Art
Most of the oxygen sensors used heretofore in detecting the concentration of oxygen in exhaust gas from internal combustion engines or other burning devices employ oxygen detecting elements made of zirconia or other solid electrolytes.
In a typical case, an oxygen sensor of this sort comprises an oxygen detecting element in a tubular form of a solid electrolyte such as zirconia which is closed at one end and open at the other end. Inner and outer conductive layers usually made of platinum are deposited on the inner and outer sides, respectively, of said oxygen detecting element. An output terminal with air holes provides communication between the atmospheric air and the atmosphere within the oxygen detecting element. A lead wire picks up an electrical output from the inner conductive layer which is electrically connected to the lead wire through the output terminal. An outer tube accommodates the oxygen detecting element and the output terminal and is provided with air holes for providing communication between the atmospheric air and the atmosphere within the oxygen detecting element. In order to detect the concentration of oxygen in exhaust gas from a burner, the sensor is attached to the burner in such a manner that the inner conductive layer is in contact with the atmospheric air while the outer conductive layer is in contact with the exhaust gas.
When the oxygen sensor having the construction described above is used as an attachment to an internal combustion engine or a burning device, the inner conductive layer in the sensor may be contaminated with the oil or water present in the environment of the engine or burner. In order to avoid this problem, a sealant that is oil- or water-tight has been inserted between the pickup lead wire and the outer tube. However, the outer tube has air holes so that oil or water can enter the sensor through these holes.